Hydrogen is used in a variety of industrial processes. Because of this demand for hydrogen and particularly for hydrogen free of impurities, an equally wide variety of techniques have been developed for separating impurities from hydrogen.
An impetus for continued improvement of hydrogen purification to levels of ultra-high purity stems from development in the manufacture of integrated circuits where ever increasing line densities require high purity processing materials. Because hydrogen is a key component in many of these semiconductor manufacturing processes, ultra-high purity hydrogen is of particular importance in this field. Commercially available hydrogen typically contains impurities, including: carbon monoxide, carbon dioxide, oxygen, nitrogen, water, and methane, among others. These components must be separated from the hydrogen to achieve appropriate levels of purity for many industrial applications.
One method for purifying hydrogen, as described in U.S. Pat. No. 5,492,682, involves a two-step process. The first step of the two part process involves the removal of carbon monoxide by contacting the stream containing carbon monoxide with a nickel catalyst to form nickel-carbonyl. This carbon monoxide-free stream is then passed through a second reaction zone wherein it is contacted with a titanium nickel catalyst in order to further purify the stream by removing methane and carbon dioxide.
Another purification method, as described in U.S. Pat. No. 4,056,373, involves the use of a selectively permeable noble-metal membrane. In this method, a membrane is selected such that only hydrogen will pass through. An example is the use of a palladium-alloy filter coil. There, the purification filter separates hydrogen from the impurities present in the hydrogen stream by limiting passage to hydrogen.
Another purification method, as described in U.S. Pat. No. 4,654,047, uses a two-step membrane/cryogenic process. The process involves a first step of separating a stream of gases into a hydrogen-rich component and a hydrogen-lean component by selective permeation through a cellulose acetate, polysulfone, or polyimide type membrane or hollow filter. The hydrogen-lean stream is subsequently treated by a cryogenic process to remove some of the remaining impurities to produce a more enriched hydrogen stream.
Still another technique for the purification of hydrogen, as described in U.S. Pat. No. 3,251,652, begins with a stream comprised of hydrogen and hydrocarbons. This stream is contacted with steam and air to convert the hydrocarbons to carbon monoxide. That stream is then treated with a gaseous diffusion process (utilizing a palladium-silver alloy membrane) to separate the hydrogen from the carbon monoxide. The stream from that process containing mostly carbon monoxide (and some hydrogen) is then contacted with steam to produce carbon dioxide and hydrogen through a shift reaction. This mixture is then passed through a second palladium-silver alloy membrane to separate the hydrogen and carbon dioxide.
Notwithstanding these various processes, there remains a need for an improved method for producing ultra-pure hydrogen, adequate for modern semiconductor manufacturing.